Polyolefins have been employed in wide applications because of their excellent mechanical properties, such as rigidity, tensile strength, and the like, chemical resistance, and processability. However, polyolefins involve difficulty in finishing, such as coating, adhesion, filling, printing, hot-stamping, plating, etc., due to their non-polarity, and many attempts have been made to overcome this disadvantage. In particular, studies have been conducted on modification of a surface layer of a polyolefin molded article by physical or chemical processes prior to finishing and put into practical application. These improved processes, however, produce insufficient effects because they require complicated steps or, depending upon etching processes, accompany deformation of molded articles. Therefore, no satisfactory results can be obtained in view of increased cost.